The present invention relates to the fixing of a geographical reference of a vehicle travelling through a pipeline.
It is now well known to inspect a natural gas transmission pipeline for structural defects, such as cracks and corrosion pits, by sending through the pipeline a so called "intelligent pig". The intelligent pig is propelled through the pipeline by the pressure of the gas so that the inspection can be performed while maintaining the gas supply. The pig incorporates means for generating a magnetic flux for establishing a magnetic field in the pipeline, and magnetic flux-sensing coils incorporated in the pig detect any irregularities in the pipeline wall, such as cracks, pits and like, which cause irregularities in the pipeline flux. Instead of or in addition to generating magnetic flux, the pig may incorporate means for generating an ultrasonic signal for transmission into and reflection from the wall. Ultrasonic sensors also incorporated in the pig sense the reflected signal which is irregular if reflected from irregularities, such as defects in the pipeline wall. The signals detected by the magnetic flux or ultrasonic signal sensors are recorded on tape and can be processed and analysed to determine the type of and extent of the defect so that the relevant defective section of the pipeline can be repaired or replaced.
Clearly it is necessary to match the appropriate defect signal to the precise location of the defect in the pipeline, in other words to fix a geographical reference or line marker of the defect. It is also a requirement that such a system should permit such matching to be made without the need to expose the external wall of the pipeline.
Normal progress of the pig through the pipe is measured by footage wheels mounted on it. While the distance measured by these wheels can be used to fix a reference for the defects, the wheels may slip, and over long distances their absolute measurement may not be accurate enough to enable an accurate fix of the defect to be made. Tees or other devices may be used to provide intermediate pipeline references, but in a region devoid of such features this is impossible.
It is therefore an object of the present invention to fix a geographical reference for the vehicle as it travels through the pipeline without the above disadvantage and without the necessity to expose the external wall of the pipeline.